Compensated density radioactive well logging tools have been in wide use in the search for subsurface hydrocarbon deposits. Typically such tools have been lowered by cable or wireline after completion of a well and prior to insertion of the casing, then drawn upwardly through the well while emitting a controlled amount of radiation. The radiation impacting on the various subsurface formations generates backscatter gamma ray radiation in inverse proportion to the density of the structure and which may be measured by the tool.
However, the level of backscatter radiation may be affected by various aspects of the well bore, subsurface formation, and of the tool, to the detriment of the accuracy of the observed density reading. The presence of filter cake coating the sides of the well bore, as well as the presence of drilling fluid within the well bore, greatly affect the measurements in that these substances differ in density from the actual subsurface formation. Moreover, inaccurate placement of the radioactive source and detection equipment within the well bore may also reduce the effectiveness of measurement operations. Various approaches have been tried in the past to overcome this distorting affect, such as to bias the radiation source and detectors to one side of the well bore thereby reducing the amount of intervening material. At the same time, multiple detectors or groups of detectors have been placed in vertically spaced relation to the radiation source, whereby the signals from the various groups may be compared and correlated, maximizing the effect of the subsurface formation and minimizing the effect of the filter cake and drilling fluid on the observed density reading.
For instance, U.S. Pat. No. 3,321,625, issued to Wahl, shows a device having a spring loaded arm which contacts the well bore at a point opposite that of the radiation source and detectors so as to position them against the well bore wall. The position of the spring biased ar within relation to the body of the tool at a given time is measured by a potentiometer. This reading is used to help interpret the measurement of formation density, since it is an indication of varations in well bore diameter and filter cake thickness at various points within the well bore. However, it will be noted that due to the size and shape of the pad containing the radioactive source and detectors, there remains a substantial amount of interfering material between the tool and the subsurface formations.
U.S. Pat. No. 4,120,353, issued to Roesner, shows a mechanical linkage apparatus which converts a powered longitudinal movement within the tool into radial extension of a pad containing the measurement equipment, as well as a resiliently biased caliper arm, as in Wahl. The linkage includes a series of bell crank arms, pivot points and cams to achieve the outward extension. However, it will be noted that in smaller diameter well bores, the retraction of the caliper arm in Roesner necessarily requires a proportional retraction of the pad. This shift seriously affects the linearity of the readings of the tool over various well bore diameters and reduces its overall effectiveness.
Therefore, it becomes desirable to construct and use a compensated density well logging tool which will eliminate or reduce the effect of drilling fluid and the filter cake on the observed density of subsurface formation and provide precise density measurements requiring less correction for a wide range of open well bore diameters.